The present invention relates to optical lenses, and more specifically, to such lenses that focus or collimate angled light entering the lens.
Currently, lenses of various types are used to collimate a diverging light beam exiting an optical waveguide and to focus light that is launched into an optical waveguide so as to more efficiently couple the light. However, a frequent concern in the optics field is the ability to focus and/or collimate light that does not enter a lens perpendicular to the plane of the lens. For example, light exiting an optic fiber may strike a bi-convex lens at non-perpendicular angle (e.g., 12xc2x0) with respect to the plane normal to the lens surface. In this case, the path of each light ray is different through the lens, resulting in a combination of aberrations including coma. Coma is one of the primary wave aberrations, characterized by an increasing size of the pattern with the off-axis distance of the object point.
FIG. 1 shows a centrosymmetric lens 10 formed from two plano-convex lenses 12 and 14. A light beam, e.g., coming from an optic fiber, is incident on lens 12 at a non-perpendicular angle to the surface of the lens. As a result, the path of each light ray is changed as the light passes through lens 12. The path of each light ray is further changed as it exits lens 14 since the angle is not perpendicular to the surface of lens 14. Since each ray traverses a path through the lens with a different optical length, the wavefront of the light beam exiting the lens is aberrated. In other words, the phase of the light is not constant across the beam. As a result, the beam exiting the lens distorts as it propagates through free-space. Consequently, when these lenses are used to couple light from one fiber to another, fiber coupling efficiency is reduced. The problem worsens as the angle relative to the lens axis increases. One solution to this problem is to tilt lens 10 such that the light enters the lens at a 90xc2x0 angle (or perpendicular) to the surface of lens 12. As a result, little or no off-axis aberrations are introduced, and the beam is properly collimated and focused by lens 10. However, tilting lens 10 requires careful and precise alignment. Further, lens 10 is typically part of a large array, which makes the alignment even less practical.
Accordingly, an optical lens is desired that can focus and/or collimate light that does not enter the lens at 90xc2x0 without the disadvantages discussed above.
According to one aspect of the present invention, a centrosymmetric optical lens is formed from two plano-convex lenses, with the planar surfaces facing each other and offset. The first plano-convex lens (i.e., the lens into which light enters) is shifted in the x-y plane so that the light enters the lens at a 90xc2x0 angle relative to the incident convex surface of the first lens. The second plano-convex lens (i.e., the lens from which the light exits) is shifted in the x-y plane so that the light exits the lens at a 90xc2x0 relative to the incident convex surface of the second lens. The result of offsetting the lens halves is that the light beam behaves the same way as if it were entering and exiting a bi-convex lens, which does not induce offaxis aberrations into the exiting beam.
In one embodiment of the invention, the two lens halves are bonded together, such as with an index-matching epoxy or by heating and fusing them together. According to other embodiments, the two lens halves are separated by a spacer, such as silicon, or by an air gap.
In accordance with another aspect of the invention, the first lens has a convex surface, while the second lens has a concave surface which is offset from the first lens. This offset meniscus lens can be formed with the convex surface having a higher radius of curvature (a positive meniscus lens) or a smaller radius of curvature (a negative meniscus lens). Other embodiments utilize a prism lens as the second lens. The prism lens is positioned or formed such that the light beam is incident to the surface of the prism lens at a 90xc2x0 angle to the surface.
Lenses and lens arrays of the present invention are easily fabricated with existing techniques without introducing off-axis aberrations, are smaller in size than arrays with similar exit diameter beams, can tolerate air gaps, and can easily handle different and large angles of incident light.
The present invention will be more fully understood when taken in light of the following detailed description taken together with the accompanying drawings.